Growth of water-borne bacteria in supply water networks and tapping points is a well recognized problem. Especially feared are the legionella bacteria that can grow rapidly to dangerous levels in the water at temperatures between 20 and 50° C. Hot water flushing or a biocide may effectively kill free-living bacteria in the water, but in the biofilm on the inner surfaces of water tanks, pipes and fittings the bacteria will often survive and start to multiply again as soon as conditions allow it. This capability of “hiding” makes certain bacteria e.g. legionella very difficult to effectively control. The problem is particularly serious in large buildings, such as hospitals, hotels and apartment buildings, the tap water systems of which have many spaces in which at times stagnant water is cooled down or warmed up to the hazardous temperature region between 20 and 50° C.
Recent attempts to address this problem have included mixers where an operator can use a special tool to temporarily flush the mixer and the whole water supply system with scalding hot water for several minutes with the aim of killing the bacteria during a round of all the tapping points in a hospital or care establishment, for example. A computer-controlled automated system for the regular flushing of tap water systems with hot water through the tapping devices has also been suggested. Further experience has shown that sanitizing effects are achieved by continuously supplying the water with oxidising biocides such as chlorine, bromine or ozone. Flushing methods are taught in e.g. U.S. Pat. No. 6,027,572 and references therein. However, all such procedures imply a great deal of manual labour and consequently high costs for the service operator in, for example, a hospital. In addition, the activities in the building will be disrupted.
Other attempts of addressing the bacteria problem is by purifying the water in, or close to, the tapping device. Methods and devices include adding ozone (U.S. Pat. No. 5,942,125), using multiple filters (U.S. Pat. No. 5,851,388), adding bactericidal agent through a pumping device (U.S. Pat. No. 5,709,546) and sterilization by UV radiation (U.S. Pat. No. 5,891,329). Although effective in certain applications e.g. in dental units, their complexity and need for maintenance make them less suitable for large-scale installation such as every tapping device in a hospital or an apartment building. Also the principle of purifying water at a late stage and not addressing the problem of the growth of bacteria in the whole tap water system could be questioned.
In U.S. Pat. No. 6,021,803, by the same applicant as in the present invention the problem of legionella bacteria is addressed by providing a tapping point including a mixer for hot and cold water, with a hot water and a cold water inlet, and a hot water and a cold water space. To impede the growth of especially legionella bacteria within the mixer it is suggested that the mixer additionally is provided with a hot water outlet from the hot water space of the mixer. The outlet is connected to a hot water return pipe and through an arrangement of valves the hot water is always kept under circulation. This will assure that the water will not cool down to the hazardous temperature region between 20 and 50° C. Thermally insulated return pipes for hot water are, in fact, normally already installed as a part of the water mains of large buildings, whereby in such cases, return pipes only need to be furnished from each tapping point to the main return pipe. This will keep the installation and maintenance cost at a reasonable level. The cold water may be circulated by a similar arrangement comprising a cold water return pipe. Spaces within the tapping point, in which it is not possible to circulate water, are evacuated whenever the tapping point is not in use.
Swedish patent nr 517,749 teaches a method and apparatus of reducing bacterial growth in a water mixer by utilizing the method of U.S. Pat. No. 6,021,803 on the hot water side in combination with a device locally circulating the water on the cold water side. If required the cold water is additionally cooled by some cooling device. The additional cooling is controlled by a temperature sensor.
The water mixer of Swedish patent nr 517,749 will, in comparison with traditional water mixers/tapping points, be complicated and typically require regular maintenance. The equipment will therefore be costly both to install and maintain. The temperature sensor and the cooling device will typically require electricity and/or gas. An electrical installation, for example, has obvious drawbacks in for example a shower.
Thus there is a demand for methods and apparatus for preventing bacteria growth that can work with conventional water mixers/tapping points as well existing supply water networks.